Robust reception of digital broadcast transmission

ABSTRACT

A method and apparatus for improving the reception of digitally modulated signals. A main signal and a supplemental signal are provided in the transmitter. The signals may be substantially identical except that the supplemental signal is advanced in time with respect to the main signal. The main and supplemental signals are sent from the transmitter to the receiver modulated on a signal. At the receiver, the supplemental signal is stored in a buffer. If the main signal is undesirably changed during transmission, corresponding portions of the supplement signal are substituted for the undesired portions of the main signal.

This application claims the benefit of U.S. Provisional Application60/(PU 010153) filed Jul. 19, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system for improving the reception ofthe signal used in digital television. More particularly, the presentinvention is useful in mobile digital television receivers.

2. Discussion of Related Art

Any terrestrial TV system must overcome a number of problems intransmitting signals to a receiver. For example, the United States hasadopted eight-level vestigial side band (8-VSB) modulation, as proposedby the Advanced Television Systems Committee (ATSC), as its terrestrialdigital television system modulation standard. The VSB system, being asingle carrier modulation system, is susceptible to fading caused bymultipath and signal attenuation. Any of the signal fading that isfrequency selective may be corrected by equalization techniques. Howeverthis can result in degraded performance when fading occurs. If the fadeis deep, wide and long enough in duration, however, the signal will belost and the demodulator system in the TV receiver will losesynchronization. Such fading is particularly severe in mobile receptionof the signal used in digital television.

The present invention seeks to overcome these problems by utilizing twosets of program material from a source in a transmitter. One of the setsis delayed in time with respect to the other. Thus, if the delayed setis used for reception and fading occurs, the set that is advanced intime can be substituted for the faded or missing portion of the signal.

While the detailed description of the current invention below focuses onthe details of the 8-VSB system, it must be recognized that the solutionof the current invention is equally applicable to any digital broadcasttransmission system that is subject to a fading channel environment.

SUMMARY OF THE INVENTION

In accordance with principles of the present invention a method andapparatus for improving the reception of digitally modulated signalsoperates as follows. A main signal and a supplemental signal areprovided in the transmitter. The signals may be substantially identicalexcept that the supplemental signal is advanced in time with respect tothe main signal. The main and supplemental signals are sent from thetransmitter to the receiver modulated on a signal. At the receiver, thesupplemental signal is stored in a buffer. If the main signal isundesirably changed during transmission, corresponding portions of thesupplement signal are substituted for the undesired portions of the mainsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a VSB transmitter incorporating theprinciples of the present invention. FIG. 1 includes sub FIG. 1A havingan MPEG Encoder and FIG. 1B having a hierarchical source encoder;

FIG. 2 is a schematic diagram of a VSB receiver incorporating theprinciples of the present invention; and

FIG. 3 is an illustration of groups of video packets received by thereceiver wherein a fade has occurred during transmission.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings and more particularly to FIG. 1A, a schematicdiagram of a transmitter incorporating the principles of the presentinvention is shown. The transmitter operates in accordance with theprovisions of the Advanced Television Standards Committee (ATSC) DigitalTelevision Standard dated Sep. 16, 1995, which is incorporated herein byreference. The digital television system includes three sections namelya source encoding and compression section a transport multiplexingsection and an RF/transmission section.

The source material is applied on an input conductor 10 to an MPEGencoder 20 which provides the source encoding and compression, typicallyin accordance with MPEG standards, e.g. MPEG-2. The source material caninclude video and audio signals, for example, which are encoded in theencoder 20 into a digital data stream. The encoding can utilize knownbit rate reduction methods and compression techniques which areappropriate for the particular signals involved. The compressed datastream provided from the encoder 20 is divided into packets ofinformation, each packet including data identifying that packet.

Also in accordance with the principles of the present invention, asecond encoder 30 is provided for the source material 10. In the encoder30 the source material is encoded into a digital packet data stream inthe same manner as in the encoder 20. However the output from theencoder 30 is applied on a conductor 31 to a packet buffer 32 whichdelays the data stream from the encoder 30 in time with respect to theoutput signal from the encoder 20. The output signal from the encoder 20is identified as the supplemental signal while the output of the encoder30 is identified as the main signal.

The output from the encoder 20 is applied on a conductor 21 to a firstinput of a transport multiplexer 40 and the output from the packetbuffer 32 is applied to a second input of the transport multiplexer 40.Additional data signals (not shown) could also be applied to themultiplexer 40, for example, control data to be utilized in the DTVreceiver. The data streams supplied to the transport multiplexer 40 aremultiplexed into a single data stream by the transport multiplexer 40.

The output of the multiplexer 40 is channel coded and modulated by thechannel coding section 50, the symbol mapping section 60, and the mixer70 utilizing the carrier oscillator 80. These circuits also insert thevarious “helper” signals that will aid the 8-VSB receiver in accuratelylocating and demodulating the transmitted RF signal. These include theATSC pilot tone, segment sync, and frame sync components.

The main signal, as it is transmitted, is shown in FIG. 3 as 310 andruns from “A” to “Z”. The alphabetic sequence represents the timeordered sequence of video packets. The supplemental signal, as it istransmitted, is shown in FIG. 3 as 300 and runs from “a” to “jj”. In theembodiment illustrated in FIG. 3, the supplemental sequence is advancedin time by more than 6 packet times, and more specifically, isillustrated in FIG. 3 as being advanced by 10 packet times.

In accordance with the principles of the present invention, the methodof transmitting two separate substantially identical signals, shifted intime is identified as “staggercasting”. Thus, FIG. 3 represents astaggercasted transmitted signal.

The main stream 310 of information and the supplemental stream 300 ofinformation can be identical except for information in each packet toidentify them. However in order to conserve channel bandwidth, the mainstream could contain data representing video and/or audio at “fullresolution” while the supplemental stream would contain reducedresolution data.

Instead of using the encoders 20 and 30 as shown in FIG. 1A it ispossible to also use a hierarchical coding method to supply the main andsupplemental channels, as illustrated in FIG. 1B. The main channel 310would be supplied with all the components but the supplemental channel300 would have only the high priority components.

FIG. 1B shows the source material being applied via the terminal 10′ tothe hierarchical source encoder 20′. The output on the conductor 21′ isthe supplemental, time-advanced, stream 300 while the output on theconductor 31′ is the main stream 310. Note that the main stream 310 isdelayed in the packet buffer 32′. In this embodiment, the supplementalchannel would have only the high priority information on conductor 21′while the main stream would include both the high priority informationfrom conductor 21′ and the low priority information from conductor 31′as combined in the multiplexer 33. The supplemental output from thehierarchical source encoder 20′ is applied to a first input of thetransport multiplexer 40 while the output from the buffer 32′ would beapplied to the second input of the transport multiplexer 40, as shown inFIG. 1A. Otherwise the transmitter functions are identical.

The use of hierarchical source coding permits the high priority data toappear in both the main and supplemental channels while all the lowpriority data is also available only in the main channel. Imagestransmitted by such a system could be displayed on mobile devices suchas personal digital assistants equipped with VSB demodulators.

Referring now to FIG. 2 a schematic diagram for a VSB receiverincorporating the principles of the present invention is illustrated. Inthe 8-VSB transmitted signal, the digital information is transmittedexclusively in the amplitude of the RF envelope and not in the phase.The eight levels of the transmitted signal are recovered by samplingonly the I-channel or in-phase information.

In the receiver shown in FIG. 2, the transmitted signal is demodulatedby applying the reverse principles that were applied in the transmitter.That is the incoming VSB signal is received, downconverted, filtered andthen detected. The segment and frame syncs are recovered. This isaccomplished by the mixer 100, the local oscillator 101, the lowpassfilter 102, the analog to digital converter 103, the mixer 104 and thecarrier recovery circuit 106 as well as the interpolator 107 and thesymbol timing recovery circuit 108, all in a known manner.

The output of the interpolator 107 is applied to the equalizer 110. Thesegment sync signal aids in the receiver clock recovery while the fieldsync signal is used to train the adaptive equalizer 110. The output ofthe equalizer 110 is applied to a forward error correction circuit (FEC)120. The error corrected signals provided by the forward errorcorrection circuit 120 are applied to and utilized in the transportdemultiplexer 130. The output from the transport demultiplexer 130includes both the supplemental stream signals on conductor 131 and themain stream signals on conductor 132. Under normal circumstances, themain stream signals are applied directly to the stream select circuit140 while the supplemental signals are applied to a packet buffer delaycircuit 150 which has a delay that matches the time period by which thesupplemental signal is advanced in the transmitter. Accordingly the twostreams applied to the stream select circuit 140 are now aligned intime.

The stream select circuit 140 normally is conditioned to pass the mainstream signals to the MPEG decoder 160. If, however, a fading eventoccurs in the received VSB signal signal, then the main stream signalswill be degraded, possibly to the point of being unusable. If the mainstream signals become unusable, then the stream select circuit 140 willbe conditioned to pass the buffered supplemental stream signals to theMPEG decoder 160. This is determined by the error detection circuit 121connected to the outputs of the forward error correction circuit 120 andthe transport demultiplexer 130.

The occurrence of a fading event can be detected by a number of possiblemeasures in the physical layer. For example, a signal-to-noise ratiodetector (SNR) may be used. This would be detected as a change inamplitude of the processed main signal. As another example, it ispossible to use a bit-error rate detector. In yet another example, it ispossible to use the undecodable error flag indication from the forwarderror correction system. When the circuit 121 determines that the mainsignal is corrupt it instructs the stream select circuit 140 to utilizethe supplemental channel data.

The use of the supplemental data will continue until either the data inthe buffer 150 is exhausted, or the receiver recovers and the mainchannel is restored to a predetermined quality threshold. It is evidentthat to be prepared for another fade in the main stream signal, once thereceiver recovers it must stay recovered long enough to permit thesupplemental packet buffer 150 to refill. The delay introduced into themain signal must be long enough to cover the expected time duration offading events while not taking a long time period to recover from suchfading events. In a preferred embodiment, the time delay introduced tothe main signal by the packet buffer 32 or 32′ in the transmitter andthe packet buffer delay 150 in the receiver may be selected to bebetween around 500 ms and a few seconds.

Also shown in FIG. 2 is a block representing a display processor anddisplay device 180 which receives the output of the MPEG decoder 160 anddevelops decoded image data for an onscreen display image to bedisplayed on the display device, and decoded sound data to be reproducedon a speaker, in a conventional manner.

Referring now to FIG. 3, an illustration is provided of thestaggercasting principles in a packet stream. FIG. 3 is a time diagramwith the groups of video and/or audio packets representing thesupplemental sequence (300) being advanced in time with respect to themain sequence (310) and, as noted above, running from “a” to “jj”. Itcan be seen that the supplemental channel 300 illustrated in the upperportion of the diagram is advanced in time by a time period “T_(adv)” ofroughly ten packets in this example.

The main channel 310 is represented by the packets “A” to “Z” in thelower portion of the diagram where packet A in the main channel 310corresponds to packet a in the supplemental channel, packet B in themain channel corresponds to packet b in the supplemental channel, and soforth. In FIG. 3 the first ten packets in the main channel 310 areindicated as zero since this is the time period by which the mainchannel 310 is delayed in the transmitter. This is the time periodduring which packets “a” to “j” are loaded into the buffer 150 in thereceiver prior to the reception of the first corresponding packet “A” inthe main stream 310. One skilled in the art will understand, however,that the main stream 310 may contain main packets corresponding topreceding packets in the supplemental channel.

FIG. 3 shows an example of a complete fade of the VSB signal in itstransmission from the transmitter to the VSB receiver. The fade beginsat time t1, and ends at time t2. After the fade, however, the circuitryin the receiver requires recovery time to resynchronize its clock to thereceived signal and reacquire error correction lock. This recovery timebegins at time t2, after the fade ends, and continues until time t3. Theillustrated fade in the packet sequences, thus, causes the loss of sixpackets from both the main 310 and supplemental 300 channels. That is,in the main channel, packets H-M are lost: packets H, I, J are lost dueto the fade and packets K, L, M are lost due to the demodulator and FECrecovery; and in the supplemental channel, packets r-w are lost for thesame reasons.

However, it may be seen that, supplemental packets h-m, corresponding tomain packets H-M, were received from time t4 to time t5, before the fadebegan and, therefore, are stored in the packet buffer 150. Because thesupplemental packet sequence 300 has been advanced by more than 6packets, which is the duration of the exemplary fade and reacquisition,the supplemental sequence h-m can be read from the packet buffer 150when the main sequence H-M is lost due to the fading event.

The system is vulnerable to fades until the supplemental buffer 150 isrepleted. This is because both the main and supplemental streams (andany others in the transport stream) were lost in the fade. Morespecifically, from time t6 to t7, the receiver receives main packetsR-W. However,. as described above, the corresponding supplementalpackets r-w were lost during the fade. Thus, there are no supplementalpackets stored in the packet buffer 150, and no protection for fades isavailable, for this time period. Full protection is available againafter time t7. Additional supplemental streams, advanced by differenttime periods, could be used to ride out multiple close successive fadesat the expense of consuming more bandwidth.

Also shown in FIG. 3 are shadings, which help to identify the processingof respective packets in the main and supplemental streams. The packetsshaded as illustrated by shading 301 are the packets decoded by the MPEGdecoder 160 at the receiver. The packets shaded as illustrated by theshading 302 are packets that are lost due to the loss of signal intransmission. The packets shaded as illustrated by the shading 303 arepackets that are lost due to receiver re-acquisition while the unshadedboxes (shading 304) are packets that are available in either the main orthe supplemental channels, but not decoded by the MPEG decoder 160.

The concept of using a supplemental signal to contain information to beprocessed during a fade event provides the same quality or a gracefuldegradation of the image. A lower quality supplemental signal requireslower throughput and less bandwidth to transmit than the full resolutionmain signals, but the lower quality image from the supplemental signalis slightly degraded from the full resolution image of the main signal.It is also conceivable to use a signal staggered in time of the samequality and even with a different compression format.

It is clear that the method and apparatus incorporating the principlesof the present invention as described above helps to correct some of theweaknesses in the VSB system or any other modulation system that issusceptible to fading in a transmission channel. The VSB system is asingle carrier modulation system and accordingly is susceptible tofading caused by multipath and signal attenuation. The use of theequalizer corrects many frequency selective fades but this is at theexpense of increasing noise in the bands when actual fading occurs. Ifthe fade is deep, wide and long enough in duration the modulator systemcan lose synchronization and the signal will be lost.

In accordance with the principles of the present invention, by having anadvance copy of the program material in memory, it is possible tocontinue demodulating by switching to the advanced (supplemental)transport system. Thus the demodulator will continue to try to recoverand If the fade is of modest duration the main stream will come back online before the stored advance stream is exhausted. When the mainprogram packets are available, the decoder will resume demodulating themainstream and begin buffering the advanced packets of the supplementalstream awaiting the next disruption in the received signal.

The described method and apparatus are particularly useful for mobilereception of the VSB signal. It is evident that mobile receivers areprone to severe fading as the receiver is moved through different areas.This can cause interruption of the received signal. As noted above, theapparatus and method according to the principles of the presentinvention provide a means of graceful degradation of this receivedprogram under temporary loss of signal due to fading.

This approach utilizes the transmission of a synchronously encoded,optionally reduced resolution, advanced set of program material from thesame source, called the supplemental signal. The technique is applicableto any streaming data but is directly useful for video and audio sincelower resolution material could be used to conserve bandwidth. As alsonoted above, this system could be particularly useful to users ofwireless personal digital assistants and entertainment digitalassistants.

While the present invention has been described with respect to aparticular embodiment and a particular illustrative example it isevident that the principles of the present invention may be embodied inother arrangements without departing from the scope of the presentinvention as defined by the following claims.

1. A method for improving the reception of transmitted digital broadcastsignals, comprising the steps of: producing a first set of programmaterial from a first source in a transmitter; producing a second set ofprogram material from said first source in said transmitter; timedelaying said first set with respect to said second set beforetransmission; transmitting the first and the second set of programmaterials on a signal for reception by a receiver; applying said firstset of program materials received in said receiver to normal receptionchannels of said receiver; storing said second set of program materialsreceived in said receiver in a buffer in said receiver; detecting anundesired change in said received first set of program materials; andsubstituting corresponding portions of said signal stored in said bufferfor any undesirably changed portions of said first set of programmaterials.
 2. A method as claimed in claim 1 wherein said first andsecond sets of program material are identical.
 3. A method as claimed inclaim 1 wherein said first set of program material is produced with adifferent quality than said second set of program material.
 4. A methodas claimed in claim 3 wherein the quality of said first set of programmaterial is higher than the resolution of said second set of programmaterial.
 5. In a receiver, a method for improving the reception ofsignals transmitted in the form of synchronously encoded main andsupplemental signals, said signals being staggered in time with saidsupplemental signal being in advance of said main signal, comprising thesteps of: storing said supplemental signal in a buffer in the receiver;processing said main signal in said receiver in a normal manner;detecting an undesired change in the processed main signal; andsubstituting corresponding portions of said stored supplemental signalfor any undesirably changed portions of said main signal.
 6. A method asclaimed in claim 5 wherein said undesired change is related to a qualityof said processed main signal and said change is detected by a qualitymeasure of said processed main signal.
 7. A method as claimed in claim 6wherein said quality measure is one or more of a signal-to-noise ratio,bit error rate or packet error rate measure.
 8. A method as claimed inclaim 5 wherein said main signal and said supplemental signal havedifferent resolutions.
 9. A method as claimed in claim 8 wherein theresolution of said main signal is higher than the resolution of saidsupplemental signal.
 10. A system for improving the reception of adigital signals comprising: means for producing a first set of programmaterial from a source in a transmitter; means for producing a secondset of program material from said source in said transmitter; means fordelaying said first set in time with respect to said second set; meansfor transmitting a signal carrying said delayed first set and saidsecond set of program material; a receiver having a first and a secondchannel for receiving said transmitted signal, said second channelhaving a buffer circuit for storing said second set of program material,and said first channel including means for processing said first set ofprogram material; a detector in said receiver for detecting anyundesired change in said processed first set; and means in said receiverfor substituting corresponding portions of said stored second set forany undesirably changed portions of said first set.
 11. A system asclaimed in claim 9 wherein said first and second sets of programmaterial are identical.
 12. A system as claimed in claim 9 wherein theresolution of said first set of program material is different from theresolution of said second set of program material.
 13. A system asclaimed in claim 11 wherein the resolution of said first set of programmaterial is higher than the resolution of said second set of programmaterial.
 14. A receiver for improving the reception of a signaltransmitted in the form of synchronously encoded main and supplementalsignals, said signals being staggered in time with said supplementalsignal being in advance of said main signal, comprising: a buffer insaid receiver for storing said supplemental signal; a signal processorin said receiver for processing said main signal in a normal manner; adetector in said receiver for detecting any undesired change in saidprocessed main signal; and means coupled to said detector forsubstituting corresponding portions of said stored supplemental signalfor any undesirably changed portions of said main signal.
 15. Apparatusas claimed in claim 13 wherein said undesired change in said main signalis a measure of the amplitude of said main signal and said detectorincludes one or more of a signal-to-noise ratio, bit error rate andpacket error.